For high temperature processing using a wafer boat in a vertical furnace, a pedestal is commonly used at a lower end of the furnace to support the wafer boat and to provide a thermal insulation plug at the bottom of the furnace. Such a pedestal typically comprises an insulating material inside a quartz envelope, with the quartz envelope supporting the wafer boat and the insulating material providing thermal insulation. Such a pedestal is described, e.g., in U.S. Pat. No. 5,601,428 to Okoshi and assigned to Shin-Etsu Quartz Products Co. In that patent, the insulating material is a quartz glass foam having an apparent density of 0.1 g/cm3 to 0.8 g/cm3.
In such pedestals according to the prior art, wafer boats are supported on the top surfaces of the pedestals. For very high temperature applications, however, the top surface of the quartz pedestal is still too hot and, therefore, too weak, to serve as a support surface. To reduce the temperature of the support surface, silicon carbide (SiC) heat shields or heat shields of other suitably heat resistant material can be placed above the quartz pedestal to insulate the top surface of the underlying pedestal from the high heat of the space occupied by the wafer boat. In very high temperature ranges (e.g., 1000° C. or greater), however, the heat shields can become so hot that they can act as radiation sources themselves. Therefore, their insulation value is limited and a large number of heat shields, occupying a large volume, would be required to sufficiently reduce the temperature of the top surface to make the top surface suitable for use as a wafer boat support surface.
To address the problem of a weak wafer support surface, it is also possible to make the envelope of the pedestal of SiC instead of quartz. Such a scheme has at least two disadvantages, however. First, SiC has a thermal conductivity that is so high that the insulating function of the pedestal would be essentially nullified. Second, SiC parts are more expensive to manufacture than quartz.
Accordingly, it is an object of the present invention to provide a thermally insulating, wafer boat-supporting pedestal that is compact and that is capable of use in very high temperature applications in furnaces, e.g., at temperatures of 1000° C. or greater.